A secondary battery is a battery capable of being repeatedly charged and discharged, and use thereof is advancing not only in an electronic device such as a cellular phone and a laptop computer but also in a field of an automobile, aircraft or the like. In response to such a growing demand for the secondary battery, researches have been actively conducted. In particular, a lithium-ion battery that is lightweight, compact and has high energy density among the secondary batteries has attracted attention from each industrial world, and has been enthusiastically developed.
The lithium-ion battery is mainly composed of a positive electrode, an electrolyte, a negative electrode, and a separator. Among the materials, as the electrode, a material prepared by coating an electrode composition on a current collector is used.
Among the electrode compositions, a positive electrode composition used for forming the positive electrode, is mainly, composed of a positive electrode active material, a conductive auxiliary agent, a hinder, and a solvent. As the binder, polyvinylidene fluoride (PVDF), and as the solvent, N-methyl-2-pyrrolidone (NMP) are generally used. The reason is that PVDF is chemically and electrically stabile, and NMP is a solvent which dissolves PVDF and has stability over time.
However, while a low Molecular weight product of PVDF has a problem of insufficient adhesion, if a molecular weight of PVDP is increased, a dissolution concentration is not high, and therefore PVDF having a high molecular weight has a problem of difficulty in increasing a solids content concentration. Moreover, NMP has a high boiling point, and therefore if NMP is used as the solvent, NMP also has a problem of requiring a large quantity of energy for volatilizing the solvent during forming the electrode. In addition thereto, an aqueous material without using an organic solvent has been recently required also for the electrode composition under a background of a growing concern for environmental issues.
In Patent Document 1, an emulsion of polyamideimide and a fluorine resin is used for replacing a solvent during preparing an electrode from NMP to water. The emulsion, however, has room for improvement in dispersibility and stability over time. Moreover, polyamideimide is used as a water-soluble polymer, in which specific examples of a component include an aromatic compound such as 4,4′-diaminodiphenyl ether, and a problem has still remained in resistance to oxidation.
Patent Document 2 discloses that cycle characteristics of a Si-based or alloy-based negative electrode are improved by forming lithium poly-γ-glutamate or the like on a negative electrode as a coating film or incorporating lithium poly-γ-glutamate or the like into the negative electrode as an additive material. Patent Document 2, however, discloses nothing on a function of lithium poly-γ-glutamate or the like as the binder in the negative electrode, in which NMP having a large load on an environment is used also in manufacturing the negative electrode containing lithium polyglutamate.